feat: Implement .env file support (#618)

Co-authored-by: Jacob Marshall <me@jhqcat.com>

crates/tuono_lib/tests/utils/env.rs

@@ -0,0 +1,7 @@
+use std::env;
+use tuono_lib::Request;
+
+#[tuono_lib::api(GET)]
+pub async fn test_env(_req: Request) -> String {
+    env::var("MY_TEST_KEY").unwrap_or("error".parse().unwrap())
+}